Non-thermal refined steels have been widely used for structural machine parts such as automobile parts from the standpoint of elimination of steps and reduction of production cost.
These non-thermal refined steels have been developed mainly for their high tensile strength (or hardness), yield strength and toughness. In this regard, as disclosed in Laid-Open Japanese Patent Application No. Sho 62-205245, for example, non-thermal refined steels have been proposed that utilize V, a typical element for precipitation strengthening. In application of such non-thermal refined steels having high strength and toughness as machine structural steel, however, the real problems are the fatigue strength and machinability.
Fatigue strength is generally understood to depend on the tensile strength and increases as the tensile strength increases. However, enhancement of tensile strength deteriorates the machinability extremely: with a tensile strength exceeding 120 kgf/mm.sup.2, production with normal efficiency will be impossible. There has thus been eager demand to develop a non-thermal refined steel by improving fatigue strength without sacrificing the machinability.
For this purpose, it is an effective means to improve durability ratio, that is the ratio of the fatigue strength to the tensile strength. In this connection, a process to reduce the high carbon isle-like martensite and the retained austenite in the structure is proposed, for example in Japanese Laid-Open Patent Application No. Hei 4-176842, by transforming the metallographic structure into a structure mainly composed of bainite.
However, despite such efforts and other development trials the durability ratio has been improved to 0.55 at most and the machinability has been improved only twice or so compared with the conventional type bainite non-thermal refined steels having extremely poor machinability.
Previously, the present inventors studied several kinds of hot forging products of metallographic structures in which a proper amount of bainite structure is mixed with ferrite structure, regarding their fatigue strength and machinability and invented a non-thermal refined ferrite-bainite type steel, usable as hot forged, having improved tensile strength and fatigue strength while keeping the machinability acceptable to the conventional machining step. This study was conducted from the three standpoints of (1) utilizing the complex precipitates as precipitation nuclei of ferrite, (2) lowering of low C and N, and (3) precipitating V carbide into a two-phase structure of ferrite+bainite. However, the steel having a bainite structure as transformed has problems of significantly lowered yield strength and yield ratio although the tensile strength and fatigue strength are improved. Due to these problems, the application, in particular, to automobile engine parts that are subjected to large load irregularity has been difficult.
The present invention is to provide a production process for a hot forged steel having high tensile strength, fatigue strength and good machinability simultaneously, which has been difficult to realize by conventional hot forging steels.